My RIB is a great little boat for a diving platform. It is only 4.2m long with a centre console so there is very little deck space.
By the time I add my esky the area was reduced further. Having all my gear on the deck was never the plan but funding held me back.
A mate who is a fabricator wanted a new speargun and a deal was struck.

I designed this the day I got the boat and it has taken me 6 months to achieve it.

I had a 12 caribe and now an 18 novurania. The thing that has saved me more space is an insulated fishbag hanging from the back of the seat (in your case could be betwen the posts of the arch (actually the arch is my next project and i want to put some canopy/bimini of off that) and the other thing that also helped was a tall but thin garbage bin where i put everything else. 3 divers have a lot of gear.

For the fins. I saw on the web a guy wh made something like a grill so that the blades could slide but not the footpockets. Maybe putting some bars on what you already have.

I was going to suggest the plastic kitchen garbage can, but polespearo beat me to it. You can have 3-4 guys fins and masks in it. You could bungee that onto the arch. I have used the bar that he spoke of that you slide your fins in but foot pockets stop. Works and keeps them from flying out and off the floor. That also could be on the arch. Make the slot about 1 1/2 inches wide.

The fin holder worked perfectly. The bungee that wraps over the toe area of the pocket locks them into place and they did not move at all. Care must be taken when loading and unloading though as you could drop one.

When I installed the rack and drilled through the transom I found wet timber. As I have a hull and deck refurbishment planned I decided to replace the transom now. That way I can refinish the deck and hull after I foam fill the hull and not have to address the same points twice. I don't want to be patching it up at a later date, do it once, do it right.

It was not too bad, a few soft spots.

Timber is mostly removed with a section under the floor to go. Access is difficult down there.

I am also going to make a new well at the back for a bilge pump. I will run a conduit from the anchor well (that is currently not drained) to the new bilge well. Thinking the well should be something small like 6 inches square. Only small but it is only to drain the deck.

This rack is awesome. It has been in some very rough seas and everything has remained very secure.
I have added nav lights and a light bar too.

If you would like some dimensions I would be happy to provide them.
I have one length of mono running top to bottom on the starboard side so the guns attach via their shark clip as additional security. The bungees will not last forever and having a secondary fixing system is a fail safe.

I also have a 10 litre jerry behind the esky and the floats are on the other side.
The plastic tub under the esky holds the weight belts but I am still looking for a solution for mask storage.

Bad photo but gives youthe idea. The light bar swivels front to back so I can use it as a squid light.:

Like all inflatables it does get some punctures. Not too many from fish spines. Debris in the water has got me a few times. Spears by some uncareful divers a few more times. But the benefits as a diving rig for me out weighs the maintenance.